Many types of catalyzer for catalyzing the exhaust from internal-combustion engines are known. They include a housing that accommodates a ceramic monolith. The monolith is honeycombed with channels. The whole surface of the monolith, including that of the channels, is coated with catalyst.
Although such catalyzers have been demonstrated reliable, they do have a drawback in that the catalyst does not eliminate pollutants while the engine is starting cold. At that moment the temperature of the exhaust is approximately 100.degree. C. The catalyzer is just coming into action and cannot operate at full capacity below approximately 400.degree. C. A motor vehicle will accordingly emit considerable pollution while it is being cold started.
A heated catalyzer is known from German 4 017 360 A1. The catalyzer illustrated in FIG. 1 includes a separate upstream heating element that parallels the entire path of flow. The heating element 4 in the embodiment illustrated in FIG. 2 parallels only part of the path. The embodiment illustrated in FIG. 3 includes a ceramic support. An electrically conductive coating 6 is sandwiched between the wall 5 of the support and an intermediate washcoat 8. To maintain a constant current through lines 12 (+V) and 7 it is absolutely essential for the adjacent areas, specifically wall 5 and washcoat 8, to be electrically insulating.
What are called start-up catalyzers, intended to eliminate the aforesaid drawback, are known (German GM 1 810 816). The catalyst is applied not to a ceramic monolith but to a metal support called a metallith. The device comprises a honeycomb of sheets of textured metal with a number of parallel channels for the exhaust to flow through. The metallith is coated with a ceramic catalyst. The metallith is heated and conducts heat to the catalyst.
The direct electrical heating of a catalyst support, however, has encountered several obstacles in that the usual metal structures are not electrically resistant enough to be employed as is for heating elements at the voltages conventionally available in motor vehicles (German Patent 563 757). Attempts have accordingly also been directed at heating the catalyzer with a separate electrical heating element (German AS 2 230 663) that does not itself act as a support. GM 8 905 073.8 discloses that 12 V of electricity can be supplied in a conventional automotive system only when the honeycomb is very complex. Such complexity of course makes the devices expensive at the very least.
Consequently both the catalyst itself and the metallith must be heated. Furthermore, since metal conducts heat very efficiently and accordingly demands considerable power, the discontinuously introduced energy will propagate very rapidly by conduction and obvious maximal temperatures will be prevented. This advantage, however, basically entails the drawback that such catalysts divert heat even in normal operation and accordingly cool off very rapidly, so that the requisite operating temperatures are soon not achieved when the exhaust is relatively cool, during idling for example (German GM 8 905 073.8).